Move the registration of decorations. (#1895)

We currently register decorations in the first pass through the
instructions.  This is a problem because the validator has not even
checked if the decoration instructions are valid yet.  This can lead to
unexpected behaviour from these side table.  For example, in
https://github.com/KhronosGroup/SPIRV-Tools/issues/1882, we use 5GB of
data to store 1 decoration for ids that are not even defined.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1882.